There was a company that produced thin flexible plastic sheets with texturing for stone and brick. I cannot remember the company or find it [my google-foo is not working well]. Does anyone know if that company is still around?
I purchased some of the same sheets both in cut stone and rough stone and brickwork at a show probably 20 years ago. They were loose sheets not in a package so no company name. I doubt if they are still in business.
I see a Plastruct 91608 that's vacuformed from thin red plastic and also a JTT Scenery Products 597420 brick sheet. I'm still racking my brain though, as I scratchbuilt a depot foundation using vacuformed brick sheet some 25 years ago and I don't think the sheets were made by these companies.
Check out Russ's offerings at The N Scale Architect as well. They have a whole line called "Building Sheets" that have proven very useful to me.